Field of the Invention
The present invention relates to a wiring substrate and a method of manufacturing a wiring substrate.
Description of the Related Art
As for a wiring substrate such as a semiconductor chip-mounting substrate including an electrode composed of Cu or a Cu alloy or a printed wiring substrate, a multilayer wiring substrate formed by a build-up method has been widely adopted to improve the function of the wiring substrate by raising the frequency of the wiring substrate and increasing the wiring density of the wiring substrate. Electronics manufacturers have competed for mounting electronic components on a wiring substrate at a high density to reduce the size, thickness and weight of a product, and a multi-pin technology and a narrow pitch technology for packaging have rapidly progressed. Specifically, technologies for mounting electronic components on a printed wiring substrate have been developed from Quad Flat Package (QFP) in the related art to Ball Grid Array (BGA) and Chip Size Package (CSP) mounting as area surface mounting.
Among the above, a flip chip-ball grid array (FC-BGA) technology in which a semiconductor chip is mounted on a printed wiring substrate with an interposer interposed therebetween and copper electrodes provided on the printed wiring substrate and the interposer are electrically connected to each other by a solder ball has attracted attention since the FC-BGA technology is available at a lower cost compared to mounting technologies by wire bonding using a gold wire.
In order to reliably connect the copper electrodes provided on the printed wiring substrate and the interposer, the interposer and the copper electrodes are subjected to surface treatment. As an example of the surface treatment, a nickel/gold plating treatment in which a surface of an electrode is subjected to nickel plating treatment and then gold plating treatment is known. In recent years, a nickel/palladium/gold treatment in which nickel plating treatment, palladium plating treatment and gold plating treatment are sequentially performed has been widely used due to high reliability of mounting technologies using a solder ball. Particularly, compared to surface treatment using an electroplating method in the plating treatment, surface treatment using an electroless plating method in the plating treatment is advantageous in that routing of wiring is not required on a wiring substrate in which wiring is arranged at a high density, and thus, the surface treatment using an electroless plating method has attracted attention.
Generally, in an electroless nickel plating bath, Pb, which acts as a trace additive and has an effect of stabilizing the plating bath, and a sulfur-based compound, which acts as a trace additive, has an effect of stabilizing the plating bath, and also, acts as an accelerator are included. These two kinds of trace additives co-precipitate Pb, S and the like in an electroless nickel-plated film formed by plating treatment, and influence the reliability of mounting technologies by solder and various properties such as selective precipitation capability or the like in the plating treatment.
The amount of Pb included in the electroless nickel-plated film by the plating treatment is several hundred ppm. As an environmental regulation regarding Pb, Restriction of Hazardous Substances (RoHS) directive has been established. However, the upper limit of Pb in the RoHS directive is 1,000 ppm, and the electroless nickel-plated film is not included as a target to be regulated. On the other hand, an environmental regulation, Joint Industry Guideline (JIG), which prevents intentional addition of Pb to a plating bath has also been established, and the environmental regulation regarding Pb is expected to become stricter in the future.
On the other hand, a solder-mounting material has gradually changed from a conventional Sn—Pb-based solder that is regulated by the RoHS directive to a solder that does not contain Pb. Specifically, a Sn—Ag—Cu-based solder such as Sn-3Ag-0.5Cu can be used.
As described above, the plated film and the solder are changed to a plated film and a solder that do not contain Pb, respectively. Therefore, it is important to ensure mounting reliability in a wiring substrate. To obtain high mounting reliability in a wiring substrate, excellent reflow resistance to endure plural times of reflow using multilayer wiring in Flip-Chip mounting or the like and excellent aging resistance to endure continuous use at a high operation temperature are required for a wiring substrate.
As a wiring substrate having high mounting reliability, a wiring substrate is proposed, including a plated film formed of a laminated film in which an electroless nickel-phosphorus plating layer containing Bi and an electroless gold-plated layer are laminated on a copper electrode (for example, see Japanese Unexamined Patent Application, First Publication No. 2000-124571).
However, since a precipitation rate of Ni is low in the electroless nickel-phosphorus plating layer, the content of P is increased. Therefore, it is difficult to obtain sufficient mounting reliability.